This invention relates to a solder mask composition and particularly to a one component, thermosetting system for use in producing screen relief images in the graphic arts and, in particular, for printed circuit boards, metal decorating and the like.
In the manufacture of printed circuit boards, the etched, printed circuit board, prior to soldering, receives a solder mask coating which is applied by a silkscreen process and cured into a protective film that encapsulates predetermined areas of the circuit boards so that only the circuit pads and selected areas of the board will accept molten solder in subsequent treatments.
Coatings which have been used for this purpose have been epoxy resins, urethane-alkyd and melamine-alkyd resin systems. The epoxy systems are objectionable because they require mixing of the hardener or catalyst with the resin shortly prior to use and have short shelf lives thereafter. Furthermore, lettering and number indexing inks adhere poorly to epoxy coatings, resulting in chipping and peeling of such inks from the printed circuit board.
The alkyd resin systems have a limited adhesiveness, particularly with high filler contents, a low resistance to scratching and to solder fluxes which impart blotchiness or a molted appearance to the product. The poor adhesiveness of these alkyd systems has, in the past, limited their use to copper relief images only and prevented their use on solder plated copper relief images.
Solder masking compositions, heretofore, have suffered from a number of disadvantages. The two component systems are objectionable because of their limited shelf life and the necessity to clean the silkscreen and other equipment immediately after use. The single component systems such as those of the alkyd resins, heretofore, have not accepted high loadings of inorganic fillers, as required to provide sufficient bodying characteristics to the coating composition. The alkyd resin systems have also not been suitable for use on solder plated copper image circuit boards and have had inadequate adhesion to withstand the hot oil washing required for removal of excess solder.